Optical compensation film, or retardation film, is known as a device for improving viewing angle of a liquid crystal display device. The optical compensation film is produced by forming an alignment film on a long web of transparent film, and then forming an optically anisotropic liquid crystalline compound layer by spreading and drying a liquid crystalline compound on the alignment film, as disclosed for example in Japanese Patent Laid-open Publication No. 9-73081.
The optical compensation film as described above may have defects caused by various factors during the manufacturing processes. The defects of the optical compensation film having the liquid crystalline compound layer are, for example, luminescent spot defects due to mixture or adhesion of extraneous matters in the liquid crystal layer, repellent defects where the liquid crystalline compound layer cannot be partially formed on the film as a support, film unevenness due to irregularity of molecular orientation, and the like. In addition, thickness unevenness of the film as the support is also the defect of the optical compensation film. Although the manufacturing processes for the optical compensation film are strictly supervised, it is hard to exterminate the defects of the produced film. For this reason, it is necessary to perform an inspection during the manufacture to locate the defective positions on the optical compensation film.
The defect inspection of the optical compensation film on the manufacturing line may be called the on-line inspection, and there are many conventional methods for the on-line inspection.
For example, Japanese Patent Laid-open Publication No. 6-235624 suggests an inspection method, wherein an inspection light beam is projected from a light source toward a target transparent film as being conveyed, and the reflected or transmitted light beam is received on a linear sensor. Based on the received light beam, fine unevenness of the film surface, extraneous matters or air bubbles in the film, or protuberances produced on an antireflective coat on the film surface can be automatically detected at a high speed.
Japanese Patent Laid-open Publication No. 8-54351 discloses a defect inspection method, wherein a high-luminous high-directional light beam is projected at an angle of 5° to 15° onto a surface of a transparent sheet as being conveyed continually, and light transmitted through the transparent sheet is captured by a camera, so as to detect the film defect based on data obtained by image-processing an output signal from the camera. This method enables detecting fine unevenness of the film thickness, i.e. unevenness of 0.1 μm to 5 μm deep and 0.1 μm to 10 μm wide.
Japanese Patent Laid-open Publication No. 11-30591 discloses a method, wherein a light source and a camera are placed in opposition to each other across a film to be inspected, and one polarizing plate is placed between the light source and the film, whereas another polarizing plate is placed between the camera and the film. The film defect is detected based on an output signal from the camera. The displacement in the polarizing direction of the polarizing plates is set to be not more than ±20°. Thereby, vertically polarized components, which are generated by irregular orientation of film molecular or slight distortion of the film, are reduced, so texture signals are lowered and local changes in transmitted light amount through the film are reduced. Then, a change in polarizing condition that occurs at a defective position is made apparent as a dark area signal.
U.S. Patent Application Publication No. US 2001/0021016 (corresponding to Japanese Patent Laid-open Publication No. 2001-324453) discloses a method, wherein a pair of polarizing plates is placed to interpose a film to be inspected. The polarizing plates are parallel to the film. An intersection angle formed between a slow axis of the film and a polarizing transmission axis of one of the polarizing plates is set to be not less than 5° and not more than 15°. In order to eliminate viewing angle dependence, an optical compensation film practically equivalent to the film to be inspected is rotated through 180° along a plane corresponding to the film surface, or reversed front side to back side, and placed between the film and one of the polarizing plates. Fine optical defects can be detected by receiving light on a CCD camera in the normal line direction of the film surface.
When the optical compensation film, especially the optical compensation film having the liquid crystalline compound layer formed thereon, has the luminescent spot defect due to mixture or adhesion of extraneous matters in the liquid crystal layer, molecular orientation of the liquid crystal layer becomes highly irregular. Therefore, the luminescent spot defect can be readily detected by placing the optical compensation film to be inspected between the polarizing plates whose polarizing transmission axes are orthogonal to each other, and photographing the film in the normal line direction of the film with a camera and the like. The repellent defect where the liquid crystalline compound layer cannot be partially formed on the transparent film can also be easily detected in the same manner. In recent years, liquid crystal display devices have higher luminous and higher definition. Along with this trend, film unevenness with low brightness that could hardly be viewed needs to be detected as an appearance defect. Such film unevenness is caused by, for example, localized misalignment of an optic axis of the liquid crystal layer, and could not be detected by photographing the film in the normal line direction thereof with a camera and the like.